That would mean Yumemi and Chiyuri are, like, ancient beings who look after the ruins, which is in fact an entrance to another dimension where science rules supreme. Reimu, Marisa, and Mima are the only ones who made it back out after getting their prizes, but the others aren't so lucky, though they still got what they want. That's also why those characters never showed up again in the series.
What if Yukari failed to mind the gap while getting on the train?